The present invention relates to a compact or lightweight cask and a production method of the cask, and to an embedded form. The cask is used for accommodating and storing spent fuel assemblies after combustion.
A nuclear fuel assembly that has combusted to the final phase of the nuclear fuel cycle can not be used more and it is called used nuclear fuel. Such used nuclear fuel contains highly radioactive materials such as FP and thus needs to be cooled thermally. The used nuclear fuel is cooled in a cooling pit in the nuclear power plant for a predetermined period of time (three to six months). After that, the used nuclear fuel is accommodated in a cask which is a shielding vessel, and transported to a reprocessing plant e.g. by a truck and then stored. In accommodating used nuclear fuel assemblies in a cask, a holding element having a grid-like sectional shape referred to as a basket is used. The used nuclear fuel assemblies in question are inserted into cells which are a plurality of spaces formed in the basket in question, whereby an appropriate holding power against vibrations and the like is ensured during transportation. In the following discussion, a cask which forms a basis for developing the present invention will be described. The cask in question, however, is explained for the sake of convenience and does not fall into so-called well-known and well-used arts.
FIG. 17 is a perspective view showing one example of a cask. FIG. 18 is a radial section view of the cask shown in FIG. 17. The cask 500 comprises a cylindrical barrel body 501, a resin 502 which is a neutron shielding member disposed on the outer periphery of the barrel body 501, an outer casing 503 thereof, a bottom portion 504 and a lid portion 505. The barrel body 501 and the bottom portion 504 are casts of carbon steel which is a gamma ray shielding material. Further, the lid portion 505 consists of a primary lid 506 and a secondary lid 507. The barrel body 501 and the bottom portion 504 are connected by butt welding. The primary lid 506 and the secondary lid 507 are fixed with respect to the barrel body 501 via bolts of stainless and the like. Between the lid portion 505 and the barrel body 501 is disposed a metal O ring which keeps the interior air-tight.
Between the barrel body 501 and the outer casing 503 is provided a plurality of inner fins 508 for enabling heat conduction. The inner fins 508 are formed of copper so as to improve the heat conductivity. The resin 502 is injected into the spaces formed by these inner fins 508 in a fluid state, and then solidified by thermosetting reaction and the like. A basket 509 has a structure such that 69 square pipes 510 are assembled in a bundle as shown in FIG. 17, and is inserted into a cavity 511 of the barrel body 501. A basket 509 has a structure that 69 square pipes 510 are assembled in a bundle as shown in FIG. 17, and is inserted into a cavity 511 of the barrel body 501.
Reference numeral 515 designates a cell for accommodating a used nuclear fuel assembly. The relevant square pipe 510 is formed of an aluminum alloy mixed with a neutron absorbing material (boron: B) so that the inserted used nuclear fuel assembly will not go critical. On both sides of a cask body 512 are provided trunnions 513 (one of them is omitted) for suspending the cask 500. Furthermore, on both ends of the cask body 512 are mounted buffer members 514 (one of them is omitted) to which lumber and the like is incorporated as a buffer material.
It is to be noted that it is desired that the above-described cask 500 is compact and lightweight from the view point of easy handling at the time of transportation and space saving at the time of storage. However, according to the configuration of the above-mentioned cask 500, since the outermost square pipes 510 line contact with the inner surface of the cavity 511 to generate a space area S between the basket 509 and the cavity 511, the diameter of the barrel body 501 becomes large and the cask 500 becomes heavy.
For addressing this problem, the only thing that is necessary to reduce the weight of the cask 500 is to make the thickness of the barrel body 501 small because the amount of radiation that leaks outside the cask is restricted by the total amount of neutrons and gamma rays. However, since the barrel body 501 is also a gamma ray shielding member, the barrel body 501 is required to have sufficient thickness to ensure the gamma ray shielding ability. Even in such a case, the thickness should fall within the range that is necessary and sufficient to shield the gamma rays. This is because excess thickness will avoid reduction of weight of the cask.
It is an object of this invention to provide a cask which is compact and lightweight, a method for producing such cask, and an embedded form.
In order to solve the above problems, according to one aspect of this invention a cask comprises: a barrel body having an integrated forged structure and which shields gamma rays; a neutron shielding member provided outside said barrel body; and a basket having a cellular structure having an angular cross section, which cellular structure comprising cells each formed with a square shaped pipe which can shield neutrons; wherein a part or the whole of the outer shape of said barrel body is matched with a shape that is formed when the vertices of the angular cross section of said basket are connected, and wherein used fuel assemblies are accommodated and stored in said cells.
According to another aspect of this invention a cask comprises: a barrel body which shields gamma rays; a neutron shielding member provided outside said barrel body; and a basket having a cellular structure having plane sections and step-like sections, which cellular structure comprising cells each formed with a square shaped pipe which can shield neutrons; wherein a portion of the outer shape of said barrel body corresponding to the step-like sections of said basket is made of a shape such that it is parallel to a line that connects the vertices of the step-like sections of said basket, and wherein used fuel assemblies are accommodated and stored in said cells.
According to still another aspect of this invention a cask comprises: a barrel body which shields gamma rays; a neutron shielding member provided outside said barrel body; and a basket having a cellular structure having a angular cross section, which cellular structure comprising cells each formed with a square shaped pipe which can shield neutrons; wherein the internal shape of a cavity in said barrel body is matched with the outer shape of the angular cross section of said basket, said barrel body having an external shape having 8 or 12 corners, wherein some or all of the sides of said barrel body are made parallel to a line which connects the vertices of the angular cross section of said basket, and wherein used fuel assemblies are accommodated and stored in said cells.
In this cask, since the outside shape of the barrel body is matched to the outside shape of the basket, the portion having excess gamma ray shielding ability is removed, so that it is possible to reduce the weight of the barrel body. Moreover, by matching the shape of the neutron shielding member provided on the outer periphery of the barrel body to the outside shape of the barrel body, it is possible to make the cask compact.
In this connection, matching the outside shape of the barrel body to the outside shape of the basket means that the outside shape of the barrel body is appropriately formed within the range assumable by persons skilled in the art, for example, so as to have a shape corresponding to only large flat portion of the outside surface of the basket; so as to have a shape similar to that defined by connecting the apexes of the square pipes constituting the basket; or so as to have a shape strictly similar to the outside shape of the basket.
According to still another aspect of this invention a cask comprises: a barrel body which shields gamma rays; a neutron shielding member provided outside said barrel body; and a basket, having an angular cross section, formed by inserting a plurality of square shaped pipes which can shield neutrons into a cavity in said barrel body; wherein a part or a whole portion of the outer shape of said barrel body is made of a shape such that it is parallel to a line that connects the vertices of the angular cross section of said basket, and wherein the internal shape of said cavity is matched with the outer shape of said barrel, said basket is inserted into said cavity, and used fuel assemblies are accommodated and stored in said cells.
According to this cask, by matching the inside shape of the cavity to the outside shape of the basket, the outer diameter of the barrel body is decreased because the space area is bridged, however, the thickness is not uniform as a result of the above, the outside shape as well as the inside shape of the barrel body is matched to the outside shape of the basket. Therefore, the portion having excess gamma ray shielding ability is removed, so that it is possible to reduce the weight of the barrel body. Furthermore, since the size of the barrel body is reduced and thus the outer diameter of the neutron shielding member can be reduced, it is possible to make the cask compact.
In this connection, the meaning of matching the outside shape of the barrel body to the outside shape of the basket is as mentioned above, and also the inside shape of the cavity involves a shape not corresponding to the outside shape of the basket partly within the range of common sense in addition to a shape corresponding to the outside shape of the basket. By designing the barrel body to have the above-mentioned shapes, the outer square pipes come into surface contact with the inner surface of the cavity, so that the decay heat generated from the spent fuel assemblies in the cells is efficiently conducted from the cask to the barrel body. Moreover, since the square pipes have neutron absorbing ability, even when spent fuel assemblies are accommodated, they will not go critical.
Further, the shape of the neutron shielding member is matched to shape formed when the vertices of the angular cross section of said basket are connected, wherein said basket having a cellular structure having a angular cross section, which cellular structure comprising cells each formed with a square shaped pipe which can shield neutrons.
Further, the outside shape of the barrel body is matched to a shape formed when the vertices of the angular cross section of said basket are connected by providing a chamfer in the part where the thickness is sufficient to shield gamma rays in said barrel body, wherein said basket having a cellular structure having an angular cross section, which cellular structure comprising cells each formed with a square shaped pipe which can shield neutrons.
That is, by matching the shape of the neutron shielding 20 member to the outside shape of the basket, it is possible to make the cask more compact due to the synergistic effect of the shape of the barrel body. In addition, by matching the outside shape of the barrel body to the outside shape of the basket, a portion results where the neutron shielding member has excess thickness, however, by matching the neutron shielding member itself to the outside shape of the basket, it is possible to suitably reduce the usage of the neutron shielding member.
In this connection, matching the shape of the neutron shielding member to the outside shape of the basket means that the shape of the neutron shielding member is appropriately formed in a manner within the range assumable by persons skilled in the art, for example, so as to have a shape corresponding to only large flat portion of the outside surface of the basket; so as to have a shape similar to that defined by connecting the apexes of the square pipes constituting the basket; or so as to have a shape strictly similar to the outside shape of the basket.
Further, the outside shape of the barrel body is matched to a shape formed when the vertices of the angular cross section of said basket are connected by providing a chamfer in the part where the thickness is sufficient to shield gamma rays in said barrel body, wherein said basket having a cellular structure having a angular cross section, which cellular structure comprising cells each formed with a square shaped pipe which can shield neutrons.
Considering the fact that if the thickness is excess to shield gamma rays, the cask becomes heavy. Accordingly, the barrel body is provided with a chamfer so long as minimum gamma ray shielding ability is ensured. As a result of this, it is possible to make the cask lightweight and compact.
Further, the outside shape of the barrel body is matched to a shape formed when the vertices of the angular cross section of said basket are connected by providing an auxiliary shielding member in the part where the thickness is insufficient to shield gamma rays in said barrel body, wherein said basket having a cellular structure having a angular cross section, which cellular structure comprising cells each formed with a square shaped pipe which can shield neutrons.
Contrary to the method in which the barrel body is designed to have excess neutron shielding ability and the portion having excess neutron shielding ability is removed, it is also possible to prepare a barrel body having such a thickness that gamma ray shielding ability is partly insufficient, and to provide the auxiliary shielding member in the part having insufficient gamma ray shielding ability. Also with such a method, it is possible to make the cask lightweight and compact while ensuring a necessary gamma ray shielding ability as the entire barrel body.
According to still another aspect of this invention a cask comprises: a barrel body having an integrated forged structure and which shields gamma rays; a neutron shielding member provided outside said barrel body; and a basket having a cellular structure having a angular cross section, which cellular structure comprising cells each formed with a square shaped pipe which can shield neutrons; wherein the internal shape of a cavity in said barrel body is matched with the outer shape of the angular cross section of said basket, wherein the outside of said barrel body is machined to a shape having a enough necessary thickness to shield gamma rays when used fuel assemblies are accommodated and stored in said cells, and wherein used fuel assemblies are accommodated and stored in said cells.
According to a method of producing a cask of still another aspect, the cask having a barrel body for shielding gamma rays and an outer casing provided on the outside of the barrel body, for potting a neutron shielding member for shielding neutrons between the barrel body and the outer casing, an embedded form is arranged in the inner surface of the outer casing beforehand, and the embedded form is removed by heating after potting of the neutron shielding member, thereby forming an expansion margin or other space portions between the outer casing and the neutron shielding member.
Such a production method is concretely used for forming the neutron shielding member into the shapes as recited in the third and fifth aspects, and also can be used for forming an expansion margin which is provided between the neutron shielding member and the outer casing. For the embedded form, a hot melt adhesive based on, for instance, vinyl acetate is used. The heating condition may be entire heating of the cask or selective heating of the embedded form. By forming the neutron shielding member in this manner, it is possible to facilitate production of the cask.
An embedded form according to a still another aspect is a form arranged inside of an outer casing provided on the outside of a barrel body for shielding gamma rays, and for forming an expansion margin or other space portions to be formed between the outer casing and a neutron shielding member to be potted, and the form is formed of a thermoplastic material and a heater is embedded in the form.
The embedded form is arranged inside of the outer casing, and the neutron shielding member is potted between the barrel body and the outer casing in this state. Next, a heater provided for the embedded form is energized, thereby melting the thermoplastic material on the periphery and removing it from inside of the outer casing. As a result, it is possible to form the expansion margin or other space portions. By using this embedded form, molding of the neutron shielding member is facilitated, so that it is possible to produce the cask easily.
Next, an embedded form according to still another aspect is a form arranged inside of an outer casing provided on the outside of a barrel body for shielding gamma rays, and for forming an expansion margin or other space portions to be formed between the outer casing and a neutron shielding member to be potted, and the form is formed by providing a thermoplastic material around a metal core and a heater is embedded in the metal core.
Since the thermoplastic material is provided around the metal core, and the metal core is heated by the heater so that only the thermoplastic material around the metal core is melted, it is possible to easily recycle the form. Accordingly, the production efficiency of the cask can be improved.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.